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Geotechnical Reports

July 4, 2011

One of the biggest questions in a construction project is what is going to happen when the shovel go’s into the ground. This is where a geotechnical report help.The geotechnical report is concerned with what is happening under the ground at your site, but it is concerned with the structural capabilities of the site only. It is not concerned with drainage, for say a septic leaching field, and it is not a Phase I (or Phase II) Site Environmental Site Assessment, which is for hazardous materials. The Geotechnical report is for foundation design and is assembled by a registered engineer.

Before we go any father we should note the geotechnical report should be given to the Architect by the Owner. This is the standard situation. However in the real word this is sometimes given to the Architect as an additional service. Really, the Architect shouldn’t take on the responsibility of  this work because it increases your liability but it is commonly done. For the purposes of the ARE assume that it is given to the Architect by the Owner at the beginning of the project.

There are several Components of a Geotech Report

  1. Project Description – A description of the existing site and what is planned for the site.
  2. Subsurface Investigation – Talk about what the found in the soils. The different types of soils and from what boring’s they came from.
  3. Foundation & Floor Slab Design Recommendation
  4. Recommendations for construction
  5. Foundation drainage requirements
  6. Compaction requirements
  7. Structural fill requirements

There are also several attachments that accompany the report:

  1. Map of site
  2. Site plan showing locations of boring
  3. Soils report logs

Before the engineer’s can write the report they have visit the site. At the site they gather core sample’s of soil. These core samples are brought back to the laboratory where they can be analyzed and developed into a report. There are also a couple of other things they are looking for when they visit the site.

  1. Water table – The level below which the subsoil and rock masses of the earth are fully saturated with water. Varies from site to site and varies seasonally.
  2. Ledge / Bedrock – The hard , solid rock at the earths surface of underlying surface soil. It can be utilized as a firm foundation for a building.

When the soil is analized at the labartory the Unified Soils Classification System is used to evaluate soils. There are three main types of soil.

  1. Course grained soils
  2. Fine grained soils
  3. Peat

Coarse Grained Soils:

Gravel – A course granular aggregate, larger than sand, formed naturally or by crushing rock. It will pass through a 3” sieve and be retained on a No. 4 sieve.

Sand – Results from natural disintegration and abrasion of rock or processing of completely friable sandstone. Granular material which passes through a ⅜” sieve, almost passes through a (no. 4) sieve, and is predominantly retained in a No. 200 sieve.

Fine Grained Soils

Clay – A fine grained, cohesive, natural mineral aggregate consisting of hydrous aluminum silicates; plastic when sufficiently wet and rigid when dry, vitrified when fired to a sufficiently high temperature.

Silt – A granular material that is nonplastic or very slightly plastic  and exhibits little or no strength when air-dried.

Peat – Fibrous Mass of Organic Matter in various stages of decomposition, generally dark brown to black in color and of spongy consistency.

Here are a few other things to remember  about soils:
Inorganic – Mineral based
Organic – Derived from plants or animals. (think composting)
Liquid Limit – The water content corresponding to a limit between the liquid and plastic states of consistency of a soil.
Plastic Limit – The lowest water content at which a soil becomes plastic.

I wouldn’t worry too much about plastic & liquid limits just understand that these help to determine how the soil will react to weight of the building upon it when there is moisture present. One other thing I would point out is liquefaction. Liquefaction is basically when the ground looses it’s bearing capacity and transforms from a solid to a liquid. This happened in the recent earthquake in Christchurch New Zealand. (see the video below) Basically I just want to point out that moisture in the ground effects it’s ability to support a building and liquefaction is a great example of it.

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